Electric current-limiting fuse



Aug. 11, 1910 .E. SALZER 3,524,157

ELECTRIC CURB IIIIIIII TING FUSE Original Filed Aug. '7. 1967 2Sheets-Sheet 1 FIG. 2

5 v; *u I' JIM MIMI! I INVENTOR:

W- 5 E. SALZER 2 ,151

- ELECTRIC CURRENT-LIMITING FUSE I Original Filed Aug. 7. 196'? 2Sheets-Sheet 3 40 BY ,4a 9

INVENTOR:

United States Patent 3,524,157 ELECTRIC CURRENT-LIMITING FUSE ErwinSalzer, Waban, Mass., assignor to The Chase- Shawmut Company,Newburyport, Mass.

Original application Aug. 7, 1967, Ser. No. 658,856, now Patent No.3,413,586, dated Nov. 26, 1968. Divided and this application July 30,1968, Ser. No. 748,679

Int. Cl. H01h 85 08, 85/10 US. Cl. 337-159 2 Claims ABSTRACT OF THEDISCLOSURE Electric current-limiting fuses include a ribbon fuse link,or ribbon fuse links, having a plurality of points of reducedcross-sectional area established by groove-like recesses and a localreduction of the thickness of the ribbon fuse link, or links, at thepoints where the groovelike recesses are located. The fuse link, orlinks, have such recesses on both surfaces thereof. The recesses formpoints of large bending compliance. The fuse link, or links, are bent atsaid points of large bending compliance to form an undulated fuse linkstructure.

This application is a division of my co-pending patent application Ser.No. 658,856, filed Aug. 8, 1967 for Electric Current-Limiting Fuse, andnow Pat. No. 3,4l3,586.

SUMMARY OF THE INVENTION Fuses embodying this invention include atubular casing of insulating material, a pair of electroconductiveterminal elements closing the ends of said casing, and a pulverulentarc-quenching filler inside said casing. A ribbon fuse link of acurrent-limiting metal, i.e. silver or copper, is immersed in saidarc-quenching filler and conductively interconnects said pair ofterminal elements. The aforementioned fuse link has a plurality ofpoints of reduced cross-sectional area and intermediate sections ofrelatively large cross-sectional area. Said plurality of points ofreduced cross-sectional area are established by points of said linkhaving a reduced thickness forming grooves extending transversely acrosssaid link and having open sides, side walls and botom walls. The latterhave a smaller thickness than the thickness of said link at saidsections of relatively large cross-sectional area.

The aforementioned plurality of grooves includes grooves at oppositesurfaces of said fuse link forming on opposite surfaces of said fuselink strips of large bending compliance, and said fuse link being bentat said points or strips of large bending compliance in accordance withsubstantially equal radii of curvature to form an undulating structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a fuselink for a current-limiting fuse embodying this invention;

FIG. 2 is a top-plan view of a fuse link according to FIG. 1;

FIG. 3 is a side elevation of the structure of FIG. 2;

FIG. 4 is in part a side elevation and in part a sec tion along IVIV ofFIG. 5 and shows a complete fuse embodying this invention; and

FIG. 5 is a section along VV of FIG. 4.

3,524,157 Patented Aug. 11, 1970 DESCRIPTION OF PREFERRED EMBODIMENT Thefuse link structure 3 of FIGS. 1 to 3 includes a plurality of grooves 4aalternatingly arranged on opposite sides of ribbon fuse link 3. Fuselink 3 is bent in opposite directions at the point of each groove 4a,thus assuming generally an undulating or zig-zag shape. The bends areestablished at points where the thickness of the fuse link is reducedand its outer fibers are close to the neutral plane of the bottomportions, or closed ends of grooves 4a. Therefore the bending stressesresulting from bending fuse link 3 to undulating or ziz-zag shape areminimized.

The points where grooves 4a are situated are points of highest currentdensity, minimal tensile strength and are subjected to high tensilestresses by magnetic action concomitant to occurrence of major faultcurrents. These stresses tend to break the link mechanically at thepoints of grooves 4a before the melting times; have elapsed which may becalculated from the melting F2 values of the fuse link structure.

Each groove 4a has an open side, side walls and a bottom wall which hasa smaller thickness than the thickness of the fuse link. Fuse link 3 isbent at each of its plurality of grooves 4a in such a direction that theopen sides of grooves 4a are wider than the bottom walls thereof. Thishas been clearly shown in FIGS. 1 and 3.

Referring now to FIGS. 4 and 5, numeral 8 has been applied therein toindicate a tubular casing of electric innsulating material. Casing 8 isclosed on both ends thereof by electroconductive terminals elements 9 inthe form of caps or ferrules mounted on the ends of easing 8. Casing 8is filled with a body of arc-quenching filler, preferably quartz sand,to which body reference numeral 10 has been applied. Terminal elements 9are conductively interconnected by an integral ribbon fuse link 3 asshown in FIGS. 1-3. Fuse link 3 is of a currentlimiting metal, i.e.silver or copper, and fully immersed in the body of quartz sand 10.Ferrules or caps 9 form recesses filled with pools 9a of solidifiedsolder, and the ends of fuse link 3 project through openings in caps orferrules 9 into the aforementioned pools 9a of solder and areconductively connected by the latter with ferrules or caps 9.

In an current-limiting fuse the dimensions of the points of reducedcross-sectional area are determined by the required fusing i t value forany particular application, i.e. by electrical requirements and are,therefore not open to choice. It is, however, possible to give variousshapes, or configurations, to the cross-section of a point of requiredcross-sectional area, and any particular configuration which is given tosuch a point has an immediate bearing upon the behavior of the fuselink. The bending stresses in a beam increase as the distance of theoutermost fibers of the beam from the neutral axis thereof is increased.The thickness of the bottom wall of grooves 4a is considerably less thanthe total thickness of link 3 and, therefore, the distance of theoutermost fibers from the neutral axis of the bottom wall of grooves 4ais very small. Because of the smallness of the above distance, thestresses set up by bending of link 3 a predetermined angle at one of itsgrooves 4a are small. In other words, the bending compliance of link 3at its points of reduced thickness is large, and it can readily be bentin response to bending forces without ever exceeding the allowable unitstress at the extreme fibers. It will be apparent from FIGS. 1 and 3that the radii of curvature at each bend or point of bend are virtuallyidentical and that all bends are virtually identical. This results fromthe displacement of the grooves On one side of the link relative to thegrooves on the other side of the link in a direction generallylongitudinally of the link.

Considering now the behavior of the bottom wall of grooves 4a when link3 is subjected to forces or twisting couples causing torsional stressingof the bottom walls of grooves 4a. There is a tendency for a fuse linkfor a fuse embodying this invention to better withstand torsionalstresses than a comparable fuse link having point-heatsource-type necksand pre-stresses resulting from punching such necks.

The melting i t value of a fuse link depends upon the material of whichit is made, i.e. its latent heat of fusion and its mean resistivity, andthe cross-sectional area F of the point where melting initiates. Hence,if a given set of conditions requires a predeterminable melting i tvalue, having selected the appropriate metal of which the fuse link isto be made, this determines the cross-sectional area of the fuse link atthe point where melting should initiate. While the cross-sectional areaF of the point of initiation of melting and arcing is thus determined,the designer of the fuse is still free to select the geometry of thecrosssection of the link where fusion and arcing is intended toinitiate. Assuming that this cross-section is rectangular, then thesides of any rectangle having the area F to achieve a required melting iis given by the equation for an equilateral hyperbola, which is:

wherein x and y are the two sides of the rectangle. The circumference orperimeter U of any such rectangle is given by the equation U=2x+2y Thecircumference becomes minimum when x=y or x/y=1 (3) and i.e. when therectangle having the cross-section F turns into a square. The larger theratio of the longer side x of the rectangle to the shorter side ythereof, the larger the perimeter of the rectangle. The larger theperimeter of the aforementioned rectangle, the larger the area ofinteraction between a pulverulent arc quenching filler as, for instance,quartz sand, surrounding the fuse link, and the region of arc initiationresulting from the flow of a fault current.

The relatively large circumference of the points of reducedcross-section in the structures of FIGS. 1 to 3 results in a relativelylarge heat flow from these points when the fuse is carrying current.Therefore, the geometry of fuse links according to FIGS. 13 tends toincrease the current rating of the fuse, or requires a smaller mass ofmetal for a given current rating. The geometry of fuse links accordingto FIGS. 1-3 further tends to result in a relatively large area ofcooling following are initiation and are extinction. This, in turn,tends to reduce the arcing i t of the fuse and to increase the coolingof the fulgurite resulting from the fusion of the surrounding quartzsand.

There are fundamental differences in regard to heat transfer during themelting time and during the arcing time of a current-limiting fuse. Itis justifiable to assume that the melting i t is a constant for anygiven fuse structure as long as the melting time is less thanmilliseconds. This constancy law results from the fact that heatdissipation from a metallic conductor in solid or liquid state isnegligible in extremely short intervals of time such as 10 millisecondsand less. Once the continuity of the metallic current path of a fuselink is broken and an are or electric gas discharge substituted for themetallic current path, the situation in regard to heat transfer isentirely different from that prevailing during the melting period of thefuse link. During the arcing period the heat transfer occurring inintervals of time in the order of microseconds may mean the differencebetween failure or success of a current interrupting device. The meltingtemperature of silver is slightly less, and that of copper slightlymore, than 1000 deg. centigrade. When an arc is kindled the temperatureof the arc path is in the order of many thousand degrees and thus thetemperature gradient is greatly increased and the area of heat flowbecomes significant. The same remark in regard to the significance ofthe area of heat flow applies also to the postarcing period ofdielectric recovery involving much smaller temperature gradients thanthe arcing period, but relatively longer times during which dielectricrecovery of the hot fulgurite is effected by heat conductionaway fromthe latter.

During the arcing period cooling and deionization is effected byrecombination of ions and diffusion of ions. Wherever there is aconcentration gradient of ions, there is a flow of ions from regions ofhigh concentration to regions of low concentration. The rate of changeof ion concentration is analogous to heat flow. Therefore the relativeincrease of the boundary area between quartz sand and the ionized metalvapors which form the arc path, and the relative increase of theboundary area between fused quartz sand aud relatively cool quartz sand,which both result from the geometric configuration of the fuse linkshown in FIGS. 1 to 4, result in an increase of heat dissipationduringand following the arcing period.

It is thus apparent that the geometry of fuse links according to FIGS. 1to 4 results in desirable mechanical performance characteristics as wellas in desirable electrical and thermal performance characteristics.

There are a number of ways for making fuse links in accordance with thisinvention of which the combination of photosensitive resists and etchingor chemical machining are the most desirable. This process is well knownin the art and, therefore, does not need to be described in detail inthis context. As a general rule, fuse links of current-limiting fusesare made of silver in which instance etching should preferably beperformed with chromium trioxide sulfuric acid solutions (see Kury, P.F., Etching Silver With Chromium Trioxide Sulfuric Acid Solutions,Journal of the Electrochemical Society, April 1956).

It will be understood that while the fuse structure illustrated hereinis a preferred embodiment of my invention, the same may take forms otherthan those specifically shown and described herein. It will be apparentto those skilled in the art that various changes and modifications maybe made without departing from the spirit of the invention or from thescope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. An electric current-limiting fuse including in combination:

(a) a tubular casing of electric insulating material;

(b) a pair of electroconductive terminal elements closing the ends ofsaid casing;

(c) a pulverulent arc-quenching filler inside said casing;

and

(d) a ribbon fuse link of a current-limiting metal immersed in saidarc-quenching filler conductively interconnecting said pair of terminalelements, said fuse link having on each of the opposite sides thereof aplurality of grooves extending transversely across said fuse link andhaving open sides, side walls and bottom walls, said bottom walls havinga smaller thickness than the thickness of said fuse link at pointsthereof remote from said plurality of grooves, said plurality of grooveson one side of said fuse link and said plurality of grooves on the otherside of said 5 6 fuse link being displaced in a direction longitudinallyReferences Cited of said fuse link and forming on opposite surfacesUNITED STATES PATENTS of said fuse link points of large bendingcompliance, v and said fuse link being bent at said points of large3,213,242 10/ 1965 Cameron 337-158 XR bending compliance inaccordance'with substantially 3, 9/ 1964 F an et al- 337295 equal radiiof curvature to form an undulating struct BERNARD A. GILHEANY, PrimaryExaminer 2. An electric fuse as specified in claim 1 wherein said 1MORGAN, Assistant Examiner link is bent at each of said plurality ofgrooves in such a direction that said open sides of said plurality ofgrooves 10 US. Cl. X.R. are wider than said bottom walls thereof. 337295

